docs: def _process_using_meta_feature_generator(self, X, meta_feature_generator):
          """Process using secondary learner meta-feature generator
  
          Since secondary learner meta-feature generator can be anything e.g. predict, predict_proba,
          this internal method gives the ability to use any string. Just make sure secondary learner
          has the method.
  
          Args:
              X (array-like): Features array
  
              meta_feature_generator (str, unicode): Method for use by secondary learner
          """
  
          all_learner_meta_features = []
          for idx, base_learner in enumerate(self.base_learners):
              single_learner_meta_features = getattr(base_learner,
                                                     self.meta_feature_generators[idx])(X)
  
              if len(single_learner_meta_features.shape) == 1:
                  single_learner_meta_features = single_learner_meta_features.reshape(-1, 1)
              all_learner_meta_features.append(single_learner_meta_features)
  
          all_learner_meta_features = np.concatenate(all_learner_meta_features, axis=1)
  
          out = getattr(self.secondary_learner, meta_feature_generator)(all_learner_meta_features)
  
          return out